1. Field of the Invention
The present disclosure generally relates to an optical scanning unit and an image forming apparatus incorporating the optical scanning unit.
2. Description of the Background Art
Generally, an image forming apparatus, such as copier, printer, or facsimile, has an optical scanning unit that scans a latent image carrier with a light beam based on image information to form a latent image on the latent image carrier. The latent image is then developed as a toner image. Such optical scanning unit (or optical writing unit) generally includes a light source, a rotatable deflector such as polygon mirror, and a driver such as polygon motor, and as a whole may be termed a polygon scanner.
Further, the optical scanning unit includes a scan lens such as an f-theta lens for converting a writing beam having an equiangular speed when deflected by the polygon mirror to a writing beam having a constant speed to be used for scanning the latent image carrier. Further, the optical scanning unit includes a reflecting mirror disposed along an optical path from the f-theta lens to the latent image carrier.
The writing beam emitted from the light source is deflected by the rotating polygon mirror at an equiangular speed, enters the f-theta lens, is converted to a writing beam having a constant speed used for scanning the latent image carrier, and is scanned in a main scanning direction. The writing beam converted by the f-theta lens may be reflected by mirrors so as to irradiate the latent image carrier.
Such optical scanning units may be encased and attached in a single casing that is hermetically-sealed with a cover so that dust or other foreign particles may not adhere to optical components such as the reflecting mirror housed in the casing.
Theoretically, an adequate latent image can be formed on the latent image carrier using only a light source, a polygon scanner, and an f-theta lens. However, in practice it is difficult to directly pass a writing beam through the f-theta lens to scan the surface of a latent image carrier in an actual image forming apparatus due to a layout configuration of the image forming apparatus. Therefore, in general, one or more reflecting mirrors are disposed along an optical path from the f-theta lens to the latent image carrier, in which the writing beam passed through the f-theta lens is reflected by the reflecting mirror and then directed onto the latent image carrier surface.
Generally, the relative positions of the light source, the polygon scanner, and the f-theta lens, which are the minimum required elements for forming an adequate latent image on a latent image carrier, is similar among virtually all optical scanning units. At the same time, however, attachment positions and attachment angles of the reflecting mirrors guiding a writing beam passed through an f-theta lens to a latent image carrier vary among optical scanning units depending on the layout configuration of image forming apparatuses.
In one related art configuration, an optical scanning unit employs a first casing and a second casing, with the first casing housing a light source, a polygon scanner, and an f-theta lens, which are the minimum required elements for forming a latent image on a latent image carrier, and the second casing housing optical devices or elements such as one or more reflecting mirrors or the like. Such first and second casings can be attached together and combined in a single optical scanning unit.
With such a configuration, the first casing, having a common configuration among optical scanning units, can be attached to the second casing having different attachment angles and attachment positions for optical elements such as the reflecting mirrors due to the layout configuration of a particular image forming apparatus.
Such a configuration having the first casing for housing the light source, the polygon scanner, and the f-theta lens, and the second casing for housing optical elements such as reflecting mirror, has following preferable features.
Specifically, parts management of light sources, polygon scanners, and f-theta lenses among a plurality of optical scanning units can be commonly conducted. Further, attachment precision testing of light sources, polygon scanners, and f-theta lenses to casings can be commonly conducted among a plurality of optical scanning units. Accordingly, optical scanning units can be manufactured with reduced cost.
In such optical scanning units, the second casing and the first casing are covered by one cover member and hermetically sealed after the first casing is attached to the second casing. A maintenance operation of polygon scanner (e.g., replacement of polygon scanner) attached in the first casing can be conducted by removing such cover member.
However, when such cover member is removed for maintenance operation of polygon scanner, optical elements such as the reflecting mirror attached to the second casing are also exposed to outside, and thereby dust may adhere to the optical elements. Such adhesion of dust or foreign particles to the optical elements may cause undesirable images, such as images having white streaks.